hca tool vs cut

[violentcrayon]

I guess I am a little confused about the HCA tool.
If all other factors were the same, what is the difference between a diamond with an 'excellent' cut and a HCA score of 1.2 and a 'Good' cut diamond with a HCA score of 1.2?

 

[treasurehunter]

Because the GIA cut grading system takes into account more things like symmetry , polish, lower and upper girdle halves and some other factors that the HCA doesn't measure.

The HCA tool should be used to eliminate stones that have poorer light return but never used as a cut grade itself.

You Should try and get a stone thats GIA excellent and has a excellent HCA score .

Do you have the cert for the good stone curious to see it .

 

[teobdl]

Difference is that HCA didn't look at the stone. GIA did, and they said that one stone was cut well ("excellent") and the other one wasn't (the "good" grade). In probably >99% of cases, it's not worth it to plug numbers of a GIA "good" stone into HCA.

 

[ame]

The HCA really just helps you weed out stones based on the basic angles that should produce the best light return. When you get into the stones that fall into the best range (0-2) you can look more at the exact angles and minutia because not all 0-2 stones in the HCA are going to be equally great performers. Ideally you should select with your eyes, if you can, within that range, because your eyes will be the ones looking at it, and your eyes have to love the stone itself, not just the paper that accompanies it. I have seen some stones that fell literally just outside the 0-2 range on the HCA that were STUNNERS, and were graded Ideal by AGS as well.

For me personally, I don't really "trust" the GIA grading system for cut yet, I don't feel like it's quite there, and I am hopeful that as it evolves, it tightens up more. And I personally would plug numbers in from a GIA Excellent or Very Good because they seem to have a really wide net for what qualifies as Excellent, and it's not always what AGS would consider as such, and when I see the actual numbers, I am left scratching my head, wondering why they gave a stone such a cut grade. I don't always plug in an AGS Ideal though, because I feel like AGS did the legwork on that already. Sometimes what gets an AGS Ideal only gets a Very Good from GIA, or GIA gives a stone an Excellent and AGS wouldn't grade it Ideal. The AGS system I do trust and will rank their cut grade over a GIA. But ultimately it comes down to seeing the actual angles, and the actual stone, before I make a final call. GIA also doesn't have an ASET, and AGS does, and that's a nice tool as well.

 

[violentcrayon]

Thanks for the replies, and I learned something!
This is an example of a good cut with high HCA score. Excellent symmetry with a good cut rating.

http://www.jamesallen.com/loose-diamonds/round-cut/0.90-carat-f-color-vs2-clarity-good-cut-sku-253199
http://cdn4.jamesallen.com/Sets/Diamond/253199/cert.jpg

 

[c-k]

Thanks for the replies, and I learned something!
This is an example of a good cut with high HCA score. Excellent symmetry with a good cut rating.

http://www.jamesallen.com/loose-diamonds/round-cut/0.90-carat-f-color-vs2-clarity-good-cut-sku-253199
http://cdn4.jamesallen.com/Sets/Diamond/253199/cert.jpg

The HCA favors stones that are shallow cuts. Although this stone is a 1.2 HCA and only a GIA good cut, it could very well be a very nice diamond. This is why you need to look at the stone and see yourself if it is a keeper. GIA is very broard on grading cut.

 

[Daisyoz]

Thanks for the replies, and I learned something!
This is an example of a good cut with high HCA score. Excellent symmetry with a good cut rating.

http://www.jamesallen.com/loose-diamonds/round-cut/0.90-carat-f-color-vs2-clarity-good-cut-sku-253199
http://cdn4.jamesallen.com/Sets/Diamond/253199/cert.jpg

My concern with that stone is the black crystal on the table and some on the sides. It may not be eye clean.

 

[ame]

Thanks for the replies, and I learned something!
This is an example of a good cut with high HCA score. Excellent symmetry with a good cut rating.

http://www.jamesallen.com/loose-diamonds/round-cut/0.90-carat-f-color-vs2-clarity-good-cut-sku-253199
http://cdn4.jamesallen.com/Sets/Diamond/253199/cert.jpg

Good cut? Or Very Good Cut? Or Excellent cut? Big difference. You really don't want "good" cut. You want the BEST cut, that should trump all the other factors. What I don't care for in this stone is the table percentage. With that depth, a smaller table would be significantly better. I don't know about that crystal, but that table number is not a number I'd be comfortable with.

 

[John P]

Thanks for the replies, and I learned something!
This is an example of a good cut with high HCA score. Excellent symmetry with a good cut rating.

http://www.jamesallen.com/loose-diamonds/round-cut/0.90-carat-f-color-vs2-clarity-good-cut-sku-253199
http://cdn4.jamesallen.com/Sets/Diamond/253199/cert.jpg

Nothing to do with symmetry here.

The HCA prefers shallower combinations than GIA does. The diamond is shallow-shallow but will have robust light return. The 13.5% crown height won't promote as much fire as optimal proportions, but it will still be extremely bright. The main problem with this combination is close-up viewing: The shallow pavilion will go dark as the viewer's head obstructs available light.

AGSL places this combination at an interesting drop-off: 58 40.6 32.5 is predicted to be AGS 1-2. 58 40.4 32.5 is predicted to be AGS 5. So, for the same reasons GIA doesn't like the idea of 40.4 32.5, AGS penalizes that combo.

It's another reason GIA's policy of averaging and rounding is confounding. We don't know whether this diamond's PA is 40.3, 40.4 or 40.5 - and don't know whether the CA average is 32.3, 32.5 or 32.7. It's the difference between predicted AGS1 and AGS5.

Grr.

 

[Texas Leaguer]

This little thread, centered around a real world example, very concisely illustrates the superiority of AGS performance cut grading vs GIA, as well as the limitations of the HCA tool.

I think the commentary distills what prosumers here mean when they say that AGS 0 “trumps” HCA score, and that it is impossible to determine whether a GIA Ex is really a top performer in the absence of additional LP analysis.

 

[treasurehunter]

Stone is also a good example of how a very low VS2 can be eye visible …

 

[Karl_K]

GIA could be right about this stone for a ring, hca could be right about it for an earring/pendant but the gia rounding makes both systems useless in this case.

Garry takes earring and pendant only stones into account and does not unfairly penalize them when properly used in the hca tool with accurate data.

edit: or what John said above.